This invention relates to antennas with special application to small, top-loaded antennas used in the military for example, for tanks, jeeps, trucks, vans, tactical command centers, helicopters and various aircraft. A serious problem exists for impedance matching these antennas over a wide frequency range. At some frequencies the antenna exhibits a complex impedance with a positive imaginary part, while at other frequencies it behaves as a negative imaginary component. To cancel out the imaginary-going portions of the complex impedance, it has been possible to construct compensating circuits to be switched on for use with the antenna. However, these compensators are useful only over a narrow range of frequencies, and a large number of different compensators is needed, each for a particular frequency band. It is noted that the switching array might have as many as 10 positions and needs considerable attention to adjust for whatever frequency happens to be in use.
This invention poses a solution to the desire for a single compensation circuit which would have the correct cancellation properties at any frequency over a very wide frequency range, 3:1, e.g. The invention makes use of a novel combination of passive circuit elements which will have the correct theoretical characteristics for these frequencies.
The major factors to be considered in the selection of the design approach to be followed are communication range and physical size of the antenna. At the present time, a height no greater than 24" and a range of at least 6 km with an RF input power level of 2w, appear to be the design goals. The discovery of the desirable impedance properties of some simple, two-element passive networks, should be useful for a wide class of antennas, from low-profile to half-wavelength dipoles. Due to its broad bandwidth, the antenna is well suited for spread spectrum, FFH, and SNAP applications.
Reference is made to the following related application: "Compact Monopole Antenna With Structured Top Load" by Donn V. Campbell, John R. Wills, and Charles M. DeSantis, Ser. No. 129,969, filed Mar. 13, 1980, now U.S. Pat. No. 4,313,121.